A mode-locked fiber laser oscillator that has a pulse energy of 10 to 100 nJ at a repetition rate of about 1 to 500 MHz without using a power amplifier can be used for various applications such as a two-photon microscope, a high-penetration optical coherence tomography, and an optical frequency comb (OFC) generator. In a mode-locked fiber laser operating in an abnormal dispersion region such as an erbium (Er)-doped fiber laser, a soliton energy Eτ (where E denotes soliton energy, and τ denotes a pulse width) is kept at a constant value due to balance between a Kerr non-linearity γ and a group velocity dispersion (GVD) D of an internal resonant device. In general, a pulse energy of each laser resonator is limited to several tens of pJ due to a limitation on a soliton area imposed by constancy of the GVD D and the non-linearity γ of the laser resonator. An Er/ytterbium (Yb)-doped fiber laser operating at 1.5 μm is an example. To generate a soliton pulse having an energy of 10 nJ or more, a laser oscillator having an amplifier or a multifilament core fiber of a large mode area is required.
Meanwhile, it has been experimentally and theoretically proved that energy of a dissipative soliton fiber laser (DSFL) having a linearly chirped output pulse can be controlled in a normal dispersion region. Similaritons and all-normal dispersion Yb-doped mode-locked fiber oscillators (YMFOs) oscillating at 1.03 μm are good examples. Furthermore, a modified soliton area theorem that provides a method for generating high pulse energy with different resonator structures having all-normal dispersion has been developed. However, it is difficult to obtain a pulse energy of 40 nJ or more using a conventional YMFO due to transition to a multipulse region.
Thus far, only a few YMFOs have been reported as having a pulse energy of 20 nJ or more. For example, an all-normal dispersion YMFO with two pump diodes, a cladding pumped double cladding YMFO, and an Yb-doped large mode area photonic crystal fiber oscillator show a pump-to-output-power conversion (POCE) of 50% or less. In particular, a ring resonator that employs a non-linear polarization rotation technique for passive mode locking should utilize discrete spectral filters to generate a mode-locked pulse. These systems utilize multiple resonant devices under a complex structure. Thus, these systems generally have a large size, and are difficult to fabricate to stably operate for a long time. OFC applications need to stably operate for a long time while having low phase noise so as to synthesize a highly stable radio frequency (RF) signal for an experiment of 9.2-GHz atomic clock transition in a cesium (Cs) atomic fountain clock.